This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The plant cell apoplast, which consists of all the compartments beyond the plasmelemma, is implicated in a variety of functions during plant growth and development as well as in plant defense responses to stress conditions. To evaluate the role of apoplastic proteins in plant development and stress response process, a 2-DE based differential proteomics approach has been used to identify apoplastic proteins and peptides involved in this process. As the amount of apoplastic proteins are at very low level, to achieve this purpose a highly sensitive differential approach is needed (such as CyDyeTM DIGE fluorescent saturation labeling) combined with high sensitivity mass spectrometric techniques. Mass spectrometry is the key technique in the downstream of proteomic research. UCSF Mass Spectrometry Facility is one of the best mass spectrometry labs in the world and has the best instrumentation and experience in this research area. By using the LTQ-Orbitrap and LTQ-FT-ICR, we can identify the proteins at very low levels. These results will enlarge our knowledge on the plant apoplast.